drivers: adc: nrfx: Use SAADC HAL for setting the burst mode
drivers: adc: nrfx: Use SAADC HAL for setting the burst mode

Replaces direct accesses to the peripheral registers structure
with the proper SAADC HAL function that appeared recently in nrfx.

Signed-off-by: Andrzej Głąbek <6788b8d556a8a447102338fc996c42180ecff44e@nordicsemi.no>

finikorg/zephyr,nashif/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,kraj/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,nashif/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,punitvara/zephyr,explora26/zephyr,kraj/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,punitvara/zephyr,explora26/zephyr,explora26/zephyr,punitvara/zephyr,punitvara/zephyr,Vudentz/zephyr,kraj/zephyr,Vudentz/zephyr,ldts/zephyr,kraj/zephyr,ldts/zephyr,explora26/zephyr,explora26/zephyr,galak/zephyr,nashif/zephyr